1. The Technology Field
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to removable and replaceable aperture cooling structures and methods of use.
2. The Related Technology
Recent advances in x-ray technology have resulted in x-ray tubes capable of producing increasingly detailed imaging and analysis results. Accordingly, x-ray generating devices have become valuable tools that are used in a wide variety of applications ranging from medicine to industrial and biotechnological testing. For example, x-rays are commonly used in diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis and testing. As such, further improvements in x-ray generating devices are continually being sought.
In typical x-ray generating devices, x-rays result when high velocity electrons are slowed or stopped by atomic forces in a target substrate. In order to generate high velocity electrons, a basic x-ray tube includes an evacuated enclosure having a filament-containing cathode and a target anode. When energized during tube operation, the filament produces a cloud of electrons by thermionic emission. The application of a voltage potential between the cathode and the anode causes the electrons to become energized and accelerate toward a target surface defined on the anode, which is axially spaced apart from the cathode and oriented so as to receive the stream of high velocity electrons.
The anode target surface is typically comprised of a material having a high atomic number such as tungsten or other heavy metals. Impingement of the stream of electrons on the target surface results in the conversion of a portion of the kinetic energy of the electrons into-photons having very high frequencies, i.e., x-rays.
Once produced, the x-rays emanate from the anode target surface and are directed through a collimating window in an outer housing containing the x-ray generating device. The collimating window allows for the x-rays to be directed toward a desired object, such as a patient's body. As is well known, the variation in the ability of x-rays to penetrate regions of different densities in the object enable various details of the object to be detected and analyzed. As such, x-rays can be used in any one of a number of applications such as, for example, x-ray medical diagnostic examination or material analysis procedures.
While a large number of electrons produced by the filament result in the creation of x-rays, some of the electrons do not produce x-rays. A percentage of these electrons strike the anode target surface and simply rebound from the surface as “backscatter” electrons. Additionally, some of the high velocity electrons emitted from the filament may stray from their intended path toward the surface. If left unchecked, some of the backscatter and stray electrons can impact undesired portions of the target surface and other interior tube components, not only creating “off-focus” x-rays that compromise the quality of the x-ray image, but producing undesired excess tube heating as well.
In order to inhibit negative consequences associated with high energy backscatter and stray electrons, some x-ray tubes include a shield structure to collect these electrons. The shield structure may be positioned between the cathode and the anode so as to enable the stream of electrons to impinge the anode target surface while preventing backscatter and stray electrons from striking the target surface and producing “off-focus” x-rays, as discussed above.
As a consequence of the high kinetic energy of backscatter and stray electrons that impact the shield structure during the tube operation, significant quantities of thermal energy are produced in the shield. Moreover, the configuration of the shield structure may result in uneven heat production and distribution. This uneven heat distribution may be exaggerated due to the manner in which the shield structure is coupled to other portions of the x-ray tube. Accordingly, shield structure regions having large temperature differentials are characterized by varying rates of thermal expansion, which result in mechanical stresses that may damage the shield or proximate regions of the x-ray device, especially over numerous operating cycles.
Because such high temperature differentials may cause destructive thermal stresses and strains in the shield structure and in other parts of the x-ray device, attempts have been made to minimize these effects through the use of various types of cooling systems. One attempt has involved x-ray tubes that utilize a liquid cooling arrangement to dissipate unwanted heat. In such an arrangement, portions of the shield structure and other tube structure are placed in direct contact with a circulating coolant, which removes heat by a convective cooling process. To maximize this convective cooling, the shield structure can be fashioned with internal cooling passages through which the coolant is circulated. This allows the shield structure to give up heat primarily by convection to the coolant flowing through its interior.
Typically, the shield structure is manufactured integrally with other components of the cathode structure. For instance, the shield structure in some x-ray tubes has been integrally manufactured with a cathode cylinder or cathode can. In other instances the shield structure is separately manufactured but then permanently affixed to the cathode can or cylinder. As a result, replacement of a shield structure integrally formed with or permanently affixed to other cathode components is difficult. Replacement can irreparably damage the shield structure, cathode structure, or both. This results in material waste and added expense during tube repair or refurbishment. Further, the fabrication costs for new cathode components and shield structures employed in tubes having a liquid cooling arrangement are typically high due to the machining required to provide the internal cooling passages in these components.
In light of the above, there is a need in the art for an x-ray tube that does not suffer from the challenges outlined above regarding cathode components. Indeed, it would be advantageous for an x-ray tube to include a cathode having a shield structure that is non-destructively separable from other cathode components should replacement of the shield structure or other cathode component be necessary, thereby decreasing the overall costs of fabricating, repairing, or refurbishing the x-ray tube.